Summary: Our laboratory, along with others, has demonstrated that myeloid antigen presenting cells (APC) have many functions previously assigned only to T helper-1 (Th1) cells. For example, macrophages and dendritic cells make interferon gamma (IFN-g) in certain settings soon following immune activation, allowing sentinel immune defenses to occur in the absence of specific recognition by T cells. We have also demonstrated that activated APC express the transcription factors, STAT4 and T-bet, initially described as lymphoid factors that promote IFN-g production. STAT4 is required for IL-12-dependent IFN-g production by APC, and T-bet mRNA levels correspond well with the levels of APC-expressed IFN-g mRNA. Each of these factors is not expressed in resting cells, but induced to high levels following various types of activating signals, including IFN-g. In this manner, both STAT4 and T-bet are central to positive feedback loops in which their expression levels are upregulated by a cytokine that they in turn induce. The primary goal of this laboratory is to investigate other roles of these two key transcription factors in the function of antigen presenting cells using a multi-pronged approach as follows: (1) Experiments with gene targeted mice: Various APC populations are being isolated from wild type, Stat4 -/-, and Stat6 -/- mice (deficient in IL-4 signaling) and evaluated for their costimulatory function (ability to induce T cell proliferation, IFN-g production, IL-4 production). Methods of adoptive transfer of APC are being developed to determine whether differences observed in vitro result in functional consequences in vivo, including responses to intracellular infection. (2) Overexpression models: Methods are being developed to overexpress Stat4 and T-bet in APC using transfection and retroviral transduction. Differences in costimulatory function will be evaluated as described in approach #1. Moreover, T-bet and Stat4 transgenic mice are being generated with a construct driven by an MHCII promoter, in which APC will overexpress these molecules. These animal models will allow evaluation of costimulatory function ex vivo and in vivo. Another goal of the laboratory is to re-evaluate the functional significance of APC production of interferon gamma. We hypothesize that it may play a role in sentinel defenses. This question will be addressed through adoptive transfer of wild type APC into IFNg -/- recipients and the use of various infection models. Moreover, we are actively collaborating with two laboratories with overlapping interests in IL-12 signal transduction, in patients with genetic immunodeficiencies (Dr. Steven Holland) and at the level of signal transduction (Dr. John O'Shea).